Typically, a controlled AC voltage is produced with indirect AC-DC-AC voltage conversion with the use of a DC link capacitor. An indirect AC-to-AC conversion uses more power switches; hence, has more loss and less efficiency than direct AC-to-AC conversion. Direct AC-to-AC conversion and removing the dc-link and its bulky electrolytic capacitor can lead to higher efficiency, lower cost, smaller size, longer life and increased reliability.
One of the most limiting factors in expanding the use of direct AC-to-AC converter-based power conditioning systems is the commutation problem. The commutation process is the procedure of turning off a main switch and turning on a freewheeling switch or vice versa. To overcome the commutation problem, several topologies have been introduced based on direct AC-to-AC conversion for an AC chopper. These topologies either use snubber circuits to avoid voltage spikes resulting from hard-switched power transistors or take advantage of resonant circuits or soft-switching (ZVS or ZCS). In some practices, selective switching based on current and voltage waveforms is proposed in which voltage or current sensors are needed to detect the polarity of the voltage and/or current.
All of the above solutions have their own restrictions. Using snubber circuits limits the output power of the converter, making it hard or impossible to use the snubber circuits at higher power levels due to the size and heat dissipation of the snubber elements. Direct AC-to-AC converters based on soft switching have some restrictions on the load power factor.
Performing selective switching patterns based on voltage/current polarity not only increases the cost, but can also affect the overall system reliability due to higher complexity and inaccuracy and may even result in failure due to noise and harmonic values especially when the amplitude of the signal is low.
Thus, there is a need for an improved direct AC-to-AC voltage converter that provides a current path in all switching states.